Method for reducing transmissions in boundary zones, in amplitude modulation transmitters operating in digital mode

ABSTRACT

Digital transmission using existing AM transmitters leads to the problem of the occurrence of unwanted out-of-band and spurious emissions. The reasons for this are that the two signals A-signal and RF-P signal, which are generated from the digital modulation signal (I/Q signal) and which are required for controlling the AM transmitter, have considerably larger bandwidths than are available in the separate branches A branch and RF branch and that the two signals arrive at the multiplier of the transmitter output stage with a delay. Using the known methods for delay compensation to below 0.3 microseconds and bandwidth reduction, for example, by a hole in the vector diagram of the I/Q signal around the 0/0 point, it is not possible to minimize the out-of-band and spurious emissions to a sufficient degree. The present method starts from the assumption that the distortions which arise because of the band limitation can be minimized using a suitable filter for the phase-modulated RF-P signal, thus also reducing the out-of-band and spurious emissions. A Gaussian filter having a linear phase response is suitable for this purpose. However, the filtered RF-P signal does not lead to success when directly fed to the transmitter output stage because the amplitude variations would be cut off again due to the switched operation. Therefore, the amplitude variations are detected from the tapped-off RF-P signal after passage through the filter using an envelope comparator and are brought together as a correction signal with the A signal in a multiplier additionally implemented in the A branch. Then, the corrected A-signal is multiplied by the RF-P signal in the transmitter output stage.

Description

[0001] The present invention relates to the field of AM broadcast transmitters (AM—amplitude modulation) used for digital broadcasting.

[0002] The usual transmitter types are non-linear AM transmitters featuring an RF branch (radio frequency) and an amplitude branch. For reasons of higher efficiency, the RF branch and the amplitude branch operate in switched mode. In the output stage of the transmitter, the RF signal and the amplitude signal are mulitplied by each other, which is also carried out in switched mode for reasons of efficiency. The switched mode is controlled by the RF signal. In contrast, for the amplitude signal, the input of the multiplier is linear.

[0003] The modulated digital signal is generated by two partial signals (I and Q), which are orthogonal to each other. The I signal (“in phase”) is modulated onto a cosine oscillation having the frequency Ft (carrier frequency). The Q signal (“quadrature”) is modulated onto a sine oscillation having the same frequency Ft. The sum of both modulated oscillations produces the complex modulated data signal (cosine 0-180degrees, sine −90-+90degrees). The modulated I/Q signal is shaped by filters in such a manner that it has exactly the prescribed curve shape with the desired bandwidth.

[0004] For digital broadcasting, the modulated I/Q signal is converted in such a manner that the two signals, amplitude signal (A signal) and phase-modulated carrier signal (RF-P signal), result therefrom which are suitable for proper control of the AM transmitter. Then, at the output of the AM transmitter, the modulated I/Q signal is generated again with higher power.

[0005] To minimize out-of-band and spurious emissions, it is required for the A signal and the RF P signal to be present in the transmitter output stage at the same time. As a result of this, the delay between the two signals has to be compensated for and the remaining difference must not be greater than 0.3 microseconds, given a channel bandwidth of 9-10 KHz, since the permissible delay difference is reciprocally proportional to the channel bandwidth.

[0006] Since both the A-signal and the RF-P signal have a considerably larger bandwidth than the I/Q signal, the increased bandwidth should be available within the transmitter, which, however, cannot be achieved to a sufficient degree due to technical and economic reasons. The insufficient bandwidths result in unwanted out-of-band and spurious emissions which have to be minimized.

[0007] From literature 1 (DE 10112025.7), it is known that the bandwidth of the A-signal and RF-P signal can be reduced if the vector diagram of the I/Q signal is provided with a hole around the 0/0 point. The larger the hole in the vector diagram of the I/Q signal, the lower are the gradients of-the out-of-band and spurious emissions in the spectrum. This connection is given by the fact that, spectrally, the gradients of the out-of-band and spurious emissions due to the delay differences correspond to the gradient of the RF-P signal, and are able to be partially corrected by the hole in the vector diagram. The hole in the vector diagram cannot be made as large as desired, because otherwise the modification of the signal appears as a disturbance.

[0008] It is also known that the shoulder distance of the out-of-band and spurious emissions depends both on the magnitude of the delay difference and on the available bandwidth in the amplitude branch and also in the RF branch of the transmitter. It follows therefrom that the delay difference should go toward zero, and that the bandwidth in the branches should be as large as possible.

[0009] Therefore, it is an object to implement as large a bandwidth as possible for both branches in order to further reduce the out-of-band and spurious emissions of the AM transmitter. For the amplitude branch, it is possible to achieve a linear phase response and, thus, a constant delay up to the frequency limit by equalization in terms of delay.

[0010] The phase-modulated RF-P signal, just as a frequency-modulated signal, has an infinite number of lateral spectral lines so that the bandwidth required for transmission would theoretically have to be infinitely large as well. Since the weighting of the spectral lines decreases rapidly, it can be achieved by using a suitable filter that only a minimum of distortions occurs because of the limited bandwidth. It is known that this condition is satisfied by a filter having a Gaussian transfer function.

[0011] The object of the present invention is to provide a method by which the influence on the frequency response that is attainable by using a Gaussian filter is used for the RF-P signal to minimize distortions due to the bandwidth limitation and, thus, to reduce the out-of-band and spurious emissions during the broadcasting of digital signals using AM transmitters.

[0012] The method starts from the assumption that a filter having a Gaussian-shaped amplitude response and linear phase is used for the RF-P signal. Due to this, the RF-P signal at the output of the filter obtains an amplitude response corresponding to this transfer function. If the RF-P signal could be multplied in this form by the A signal, there would be a reduction in the out-of-band and spurious emissions and the desired result could be accomplished quite easily. However, since the transmitter output stage operates in switched mode for reasons of efficiency, the amplitude response of the RF-P signal achieved by the Gaussian filter would be cut off again.

[0013] Therefore, the following method is used to achieve the objective. The RF-P signal is tapped off from the RF branch and passed through a filter having a Gaussian-shaped amplitude response and linear phase. At the output of the filter, the modified RF-P signal has an amplitude response corresponding to the Gaussian transfer function. These amplitude variations are detected using an envelope comparator so that the envelope signal is available as a correction signal by which the A signal is multiplied in a multiplier additionally implemented in the A branch. The corrected A signal resulting from the multiplication reaches the linear input of the transmitter output stage while the RF-P signal is present in its original form at the switched input of the transmitter output stage (see FIG. 1). The multiplication of these two signals produces the digital broadcast signal which, because of the correction method, causes considerably less out-of-band and spurious emissions.

[0014] The described method leads to the desired result because, first of all, the multiplier of the transmitter output stage allows linear operation for the A-input without reduction in efficiency and, secondly, the associative law applies to a product, the associative law being used here as follows:

r*a=(l*k)*a=l*(k*a)

[0015] r—filtered RF-P signal (with amplitude variations)

[0016] a—amplitude signal

[0017] l—unfiltered RF-P signal (constant amplitude)

[0018] k—amplitude variations of the filtered RF-P signal=correction signal

[0019] List of Reference Numerals Used

[0020]1. Gaussian band pass filter

[0021]2. envelope detector

[0022]3. multiplier

[0023]4. transmitter power output stage

[0024] A-signal amplitude signal

[0025] RF-P signal phase-modulated RF (radio frequency) signal 

What is claimed is:
 1. A method for reducing out-of-band and spurious emissions of AM transmitters in digital operation, in which the amplitude signal for controlling the linear input of the transmitter output stage and the phase-modulated RF-P signal for controlling the switched input of the transmitter output stage are produced from the digital modulation signal, and the amplitude signal and the RF-P signal are multiplicatively combined in the transmitter output stage, wherein for the RF-P signal, the distortions which arise because of the band limitation in the RF branch and which result in unwanted out-of-band and spurious emissions are reduced using a suitable filter; a Gaussian filter having a linear phase is used as the filter for the phase-modulated RF-P signal; the RF-P signal is tapped in the RF branch and passed through the Gaussian filter (1) in a separate branch; the filtered RF-P signal is fed to an envelope detector (2) to detect the amplitude variations; the detected amplitude variations are combined as a correction signal with the amplitude signal in a multiplier (3) additionally implemented in the A branch; and the corrected amplitude signal is fed to the linear input of the transmitter output stage (4) and multiplied by the RF-P signal in the transmitter output stage. 